diff --git a/setup.py b/setup.py
index 3b6b11ea..603aa34a 100644
--- a/setup.py
+++ b/setup.py
@@ -83,6 +83,13 @@ requires = [
 
 extras = {
 
+    'docs': [
+        #
+        # package                       # low                   high
+
+        'Sphinx',                       # 1.2
+        ],
+
     'tests': [
         #
         # package                       # low                   high